Referring to FIG. 7, a conventional U-type slag-tap firing boiler includes a combustion furnace 105 including a combustion chamber 101 having water-cooled walls coated with a refractory liner of a refractory material, burners 102 installed to the ceiling of the combustion chamber 101 in a vertical position, a slag-tap 103 formed at the bottom of the combustion chamber 101 to discharge molten slag, and a slag screen 104 formed by arranging multiple screen tubes 104a. The slag screen 104 is disposed in a sectional shape shown in FIG. 8 at a position, where flames flowing downward through the combustion chamber 101 start flowing upward, of the combustion chamber 101. The boiler also includes a convection heat transfer unit 107 including a radiant furnace 106 having exposed steel walls and disposed below the combustion furnace 105 with respect to the flowing direction of flames, and superheater tubes.
The slag screen 104 separates the combustion furnace 105 and the radiant furnace 106 to prevent temperature drop in the combustion chamber 105 due to heat transfer by thermal radiation from the combustion furnace 105 into the radiant furnace 106, and to reduce load on downstream members by arresting ash contained in the combustion gas. The slag screen 104 is an essential component of the U-type slag-tap firing boiler for low-NOx operation. Moreover, a slag quenching water bath 108 provided with a slag conveyor 109 therein, a pressure-measuring nozzle 110 installed to the combustion chamber 101, a pressure-measuring nozzle 111 installed to the radiant furnace 106, and tertiary nozzle 112 installed to the combustion chamber 101 to stage air into the combustion chamber 101 for two-staged combustion are shown in FIG. 7.
The refractory liner of the combustion furnace 105 coats a portion of the inner surface of the combustion furnace 105 extending from a part around the burners 102 through the slag screen 104 to an inclined part of the combustion chamber 105 below the slag screen 104. Coal ash deposited on the refractory liner melts in molten slag, the molten slag flows and is maintained at high temperatures around the fluid temperature. The thickness of a slag layer formed on the inner surface of the combustion furnace 105 changes according to the fluid temperature of the coal ash or the hemispherical temperature. Thus, the thickness of the slag layer is dependent on coal characteristic or on load.
Techniques relating to this conventional U-type slag-tap firing boiler are mentioned in U.S. Pat. No. 6,058,855.
The following methods have been taken in an attempt to achieve the low-NOx operation of conventional U-type slag-tap firing boilers.
(1) Recirculation of the exhaust gas
(2) Blowing air into the combustion furnace for a tertiary combustion in addition to air blown into the combustion furnace by the burners
(3) Reduction of the particle size of pulverized coal
(4) Fuel reburning
Techniques relating to (2) blowing air into the combustion furnace for a tertiary combustion will be explained. It is known that low-NOx operation can be achieved by reducing burner air ratio. For example, the conventional U-type slag-tap firing boiler shown in FIG. 7 is reducing the rate of supply of air to the burners 102 such that burner air ratio is on the order of 0.8. However, if burner air ratio is reduced to about 0.8, the amount of heat generated in the combustion furnace 105 decreases by about 30%, the temperature in the combustion furnace 105 drops about 100° C., and the thickness of the slag layer is multiplied by 1.5 to 1.6. Consequently, the temperature of the discharged slag drops, the slag cannot be stably discharged, the deposition of the slag on the screen tubes 104a of the slag screen 104 increases, the apparent outside diameter of the screen tubes 104a coated with a slag layer increases, and clinker grows in part of the screen tubes 104a, thereby, continuous operation becomes difficult. Therefore, when burner air ratio for the conventional U-type slag-tap firing boiler is reduced to about 0.8, air needs to be blown into a space above the slag screen 104 for second-stage combustion to maintain air ratio at the slag screen 104 at 1.
In the conventional U-type slag-tap firing boiler, air needs to be blown into the space above the slag screen 104 for second-stage combustion to maintain air ratio at the slag screen 104 at 1 for the complete combustion of coal in order to prevent plugging of the slag-tap 103 for discharging molten slag due to temperature drop in the combustion furnace and plugging of the slag screen 104 due to clinker growth on the screen tubes 104a. Consequently, it has been difficult to achieve satisfactory low-NOx operation. The NOx concentration at the exit of the U-type slag-tap firing boiler is in the range of 400 to 500 ppm (in case of 6% O2) at the lowest when techniques relating to the methods (1), (2) and (3) are used in combination, and 150 ppm (in case of 6% O2) at the lowest when techniques relating the methods (1), (2), (3) and (4) are used in combination. Thus, a NOx removal system needs to be connected to the exit of the boiler to keep a limit NOx concentration prescribed in air pollution control laws.
The amount of NOx, i.e., a pollutant that causes environmental pollution, is dependent on an oxidizing atmosphere or a reducing atmosphere bounded by an air ratio of 1, and combustion temperature. The amount of NOx, is larger at higher temperatures in an oxidizing atmosphere, while the amount of NOx is smaller at higher temperatures in a reducing atmosphere. The amount of NOx produced in an oxidizing atmosphere is several tens to several hundreds times that of NOx produced in a reducing atmosphere at 1400° C. nearly equal to fluid temperature.
While the U-type slag-tap firing boiler is in operation, the pressure in the radiant furnace 106 is controlled by an induced draft fan disposed at the point where gases leave the U-type slag-tap firing boiler so that pressure measured at the pressure measuring nozzle 111 of the radiant furnace 106 is in the range of −0.1 to −0.2 kPa. Pressure measured at the pressure-measuring nozzle 110, i.e., combustion air pressure, is monitored. The difference between the pressure measured at the pressure-measuring nozzle 110 and that measured at the pressure-measuring nozzle 111 correspond to a pressure loss caused by the slag screen 104. Pressure at the pressure-measuring nozzle 110 varies with the thickness of the layer of slag deposited on the screen tubes 104a of the slag screen 104, and is dependent on the characteristic of coal and the load.
It is decided that the slag screen 104 has been plugged when the pressure at the pressure-measuring nozzle 110 increases. However, since the pressure at the pressure measuring nozzle 110 is dependent on the characteristic of coal or the load as mentioned above, it is difficult to detect the plugging of the slag screen 104. Since the pressure increases gradually, the slag screen 104 is heavily plugged and the U-type slag-tap firing boiler reaches a serious state where the further operation is impossible before it is known that the slag screen 104 has been plugged.
The present invention has been made to solve those problems in the conventional U-type slag-tap firing boiler in view of the foregoing NOx generating characteristic of coal combustion, and it is therefore an object of the present invention to provide a U-type slag-tap firing boiler capable of stably maintaining discharging molten slag produced by melting coal ash and of operating at a very low NOx emission even if the U-type slag-tap firing boiler is provided with small-capacity NOx removal system or not provided with any NOx removal system, and method of operating the U-type slag-tap firing boiler. Thus, the present invention is intended to reduce the equipment cost and running cost of the U-type slag-tap firing boiler. The method of operating a U-type slag-tap firing boiler according to the present invention is intended to detect the plugging of the slag screen accurately in a short time, to clean the plugged slag screen and to continue safety operation.